It may be due to allergic reaction or infection. Is the swollen part fiery red and painful on touch? If so it must be due to infection, which may need oral antibiotics in addition to local application of antiseptic cream.
it may be due to allergic reaction or some insect bite
as vagina lip is very sensitive part ,mild trauma can lead to swelling
apply antibiotic cream three times a day
take tab cetrizine at night
avoid sex for a wk
if u dont fell well in 3 day ,better to consult doctor